1. Field of the Invention:
This invention relates to exposure control systems for cameras, and more particularly to an improvement in stabilization of the exposure control system against variation of the ambient temperature for deriving reliable and accurate exposure values as needed for low scene luminance range.
2. Description of the Prior Art:
Conventional exposure control apparatus utilize a photoconductive cell as a light-receiving element. The photoconductive cell is characterized as having a limited linear range of its luminance versus output current signal response characteristic, and the response in a low current range is substantially in error. Accordingly, this type of exposure control apparatus cannot be used directly on very small current signals as might be obtained when photographing subjects having very low luminances. It has been suggested that by using a photovoltaic cell such as a blue cell and photodiode as a sensor, the linear response range can be extended toward the lower limit. In this case, however, photovoltaic cells, which generally provide gentle slopes of ramp of the output voltages with respect to the scene luminance, require operational amplifiers for association therewith to produce sufficient high levels of the signal voltages. The use of operational amplifiers in an exposure control system creates an alternative difficulty of stabilizing the system against variation of the ambient temperature due to the temperature dependences of amplification of the operational amplifiers. To ignore instabilities of the operational amplifiers is to limit the accuracy of exposure control by which correct exposure would be otherwise made.